As the world prepares for potential year-2000 computer glitches, the Department of Defense is cautioning users of its satellite-based Global Positioning System (GPS) to prepare for a different kind of date rollover that will occur on August 21st. This problem could afflict incoming satellite data and end up providing inaccurate information to GPS receivers.

GPS works with a group of satellites that orbits the earth and allows sailors with ground receivers to pinpoint their exact location. The GPS system calculates time by counting the number of lapsed weeks since January 6, 1980. At midnight on Saturday, August 21, that counter will roll back to zero weeks.

While the Air Force, which operates the satellites out of Schriever Air Force base in Colorado, says the rollover won't create problems for the satellites or its ground control center, individuals with ground receivers may encounter some navigation problems if they don't get their systems upgraded, the Air Force warns.

One of the glitches that could occur is a sudden jump in the receiver's navigation position, which should level off and right itself. The receiver could also begin drifting slowly from its proper navigation position. This problem can be corrected by turning the equipment off and then on again.

The Air Force is urging GPS users to contact their receiver manufacturers to make sure the technology is "end-of-week (EOW) rollover compliant." The GPS EOW rollover happens every 1,024 weeks -- about once every 20 years. The GPS system calculates time by counting the number of weeks since January 6, 1980 -- up to a maximum of 1,023 weeks. At midnight between August 21-22, the GPS week counter will roll back to zero weeks. The Defense Department says this will not create problems for the GPS satellites or DOD's GPS ground control center, but it could be a problem for consumers who use GPS receivers and related applications. That's because after August 21, receivers could process satellite data incorrectly and display inaccurate information.

As for the year-2000 date change, the Department of Defense has determined that the GPS satellites and its ground control center will operate properly after December 31, 1999.

But if GPS receivers and applications are not Y2K ready, they could process satellite data incorrectly. Specifically, navigators may experience one of the following problems with their receiver:

It will be unable to locate the satellites, resulting in the receiver not working at all.

It will take more time than usual to locate the satellites.

It will appear to be working but display inaccurate positions, times or dates.

What should you do? If you use GPS, check with your receiver manufacturer to find out if it is EOW-rollover- and Y2K-compliant. You may want to check the Coast Guard Navigation Center's Web site where the Department of Transportation has posted a list of receiver manufacturers and contacts. You also can call the free Y2K consumer hotline at (888) USA-4-Y2K) to get manufacturer contact information. You will need to tell the manufacturer your receiver's model, serial number and the firmware version or release date that is displayed on the startup screen.

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